Here is a stage direction from the script for Sleeper:
EXT. LONDON-DAY-ESTABLISHING
The bustling city. Sure, what the hell. Haven’t we seen enough of the countryside? And if nobody recognizes it, super in one of those “London, England” title cards. That’s the ticket. Anyway…

The following lines were cut from the episode Never Leave Me:
Xander: “I know. You were in the wrong place at the wrong time. I get that. But they don’t. They need your answers. I does seem suspicious, you know, you buying the blood.”
Andrew: “That blood wasn’t for me. I …

Kevin Daniels, who played the bouncer who Buffy speaks to in Sleeper, has also been in Frasier, JAG, Law & Order and the movie Kate & Leopold.

The Seal of Danthazar is a circular metal object which covers a portal over the Hellmouth, located under Sunnydale High. It is activated by a ritual involving blood letting and a knife inscribed with Tuaric: “The blood which I spill, I consecrate to the oldest evil.” Andrew stabbed and killed …

The episode Sleeper starts exactly where Conversations with Dead People finished. Everyone is in the same clothes, and we find out the reason why Xander wasn’t in the last episode (he says he sometimes sleeps at night). Meanwhile, Spike buries the woman he killed at the end of that episode.

Rob Nagle, who played the Watcher in London in Sleeper, appeared in American Pie: The Wedding, which starred Alyson Hannigan. He played the ‘Floral Assistant’.

The following line was in the original script for the episode Sleeper:
Anya: “I… am here, obviously, because of the reason which I am about to tell you, with the following words, and that reason is…ush, clearly and obviously too, um… sex.”

In Conversations with Dead People, Dawn is confronted by an evil spirit in her house, which seems to be trying to stop her mother from contacting her. The spirit is most probably the First attempting to torment her. She performs a casting spell to get rid of the spirit, and …

R. J. Brooks was a jock student at the new Sunnydale High. He owned a magical letterman jacket which made women fall for him (though he didn’t seem to be fully aware that it was the jacket that was doing this). Dawn fell for R. J. when she saw him …

In Lessons, manifest spirits haunt Buffy, Dawn and the others. A manifest spirit is like a ghost but is able to take a physical form such as (in this case) rotting dead bodies. Manifest spirits are raised from the grave using a talisman - made of bone, twigs, feathers and …

When Tara is talking to Willow at the end of the episode Entropy, Willow’s dress from Once More, With Feeling can be seen hanging on the door.

In the season four episode Where the Wild Things Are, Spike and Anya talked about getting revenge on Drusilla and Xander. They also bonded over the loss of their respective powers. They abandoned the ‘revenge’ idea and instead Anya took Spike to a party. Spike “comforted” Anya again in Entropy …

Marti Noxon said about the episode Entropy: “The Spike-Anya thing really worked. Those two characters have a real chemistry together and it’s fun to say, ‘Oh, that works.’ Also we were just trying to put the knife in a little bit deeper.”

Buffy and Dawn go shopping in Entropy, as Buffy tries to make up for Dawn for nearly killing her. The scene was filmed on the Santa Monica promenade in L.A. It’s near Venice Beach.

Xander’s old feelings for Buffy come out in the episode Entropy, when he discovers that Buffy has been sleeping with Spike. His feelings for the Slayer haven’t been mentioned for a while, but he was obsessed with her in the first couple of seasons.